Due to fuel regulations, there is a need for weight reduction of a vehicle. Therefore, a lightweight composite material has been applied to vehicle shells, and in particular, a roof.
For combining the roof using this lightweight composite material and a vehicle body, mechanical joining is used and includes bolt fastening or riveting, and adhesive bonding is also used. Specifically, in the mechanical bonding method, an operator may insert a steel flange to a composite material roof for molding and then weld the flange to a vehicle body, but this has disadvantages that molding becomes complicates and weight may be increased by the steel part. Further, there is a method of perforating at the flange part and then bolting or riveting the flange to a vehicle body, but the process may become complicated because the perforation is added, and corrosion may occur due to a potential difference at the fastened region.
However, the adhesive bonding method has benefits in that it does not need a hole drilling process for fastening, weight is generally not increased, watertight properties are good, and load can be distributed over a large area. However thermal expansion with a material to be adhered should be considered, because a Coefficient of thermal expansion (CTE) of the adhesive may be different from that of the steel vehicle body and the composite material roof. Therefore, there is a disadvantage that stress may occur according to temperature changes of an external environment. Further, because electric conduction between the materials to be adhered becomes difficult, there is a disadvantage in that it is unfavorable to roof electric conduction of an electrostatic painting or electric conduction through an outside of a vehicle body when lightning occurs or strikes.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.